Method of fabricating semiconductor device

ABSTRACT

There is provided a method of fabricating semiconductor devices that allows ion implantation to be performed at high temperature with ions accelerated with high energy to help to introduce dopant in a semiconductor substrate, in particular a SiC semiconductor substrate, at a selected region to sufficient depth. To achieve this the method includes the steps of: providing the semiconductor substrate at a surface thereof with a mask layer including a polyimide resin film, or a SiO 2  film and a thin metal film; and introducing dopant ions.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a 371 of PCT/JP04/05649 filed on Apr. 20, 2004,which claims benefit of Japan application 2003-122240 filed on Apr. 25,2003, the contents of which are hereby incorporated herein by referencein their entirety.

TECHNICAL FIELD

The present invention relates generally to methods employing ionimplantation to provide a semiconductor substrate at a surface thereofwith a doped region to produce a semiconductor device, and particularlyto such methods that include providing a SiC semiconductor substrate ata surface thereof with a mask used for ion implantation, and thenintroducing dopant ions.

BACKGROUND ART

Silicon carbide (SiC) provides a wide bandgap, and a maximum insulatingelectric field larger than silicon (Si) by approximately one digit.Furthermore, SiC is comparable to Si in carrier mobility and comparableto GaAs in electron saturation drift rate, and also large in dielectricstrength. As such, SiC is a material expected to be applied to rapidswitching devices or devices for large power and similar,next-generation semiconductor device for electric power (in particular,junction field effect transistor (JFET), and the like).

SiC's crystal structure includes hexagonal close packed structure andcubic close packed structure, and the former structure further includesa large number of structures different in layer repetition cycle andmore than 100 types of poly-type are known. Representative poly-typesare for example 3C, 4H, 6H and the like. “C” represents cubic crystaland “H” represents hexagonal crystal, and a preceding numeral representsa repetition cycle. For cubic system, 3C is the only one, and it isreferred to as β-SiC while the others are generally referred to asα-SiC.

Recently, Schottky diode, vertical MOSFET, JFET, thyristor and the like,or CMOS-IC, the most general-purpose semiconductor device, areprototyped as devices for electric power and their characteristicssuggest that they have a potential to implement significantlysatisfactory characteristics as compared with conventional Sisemiconductor devices.

While SiC-vertical MOS semiconductor device, SiC-JFET device and thelike are expected to implement significantly excellent characteristics,in reality, however, there is only a small number of reports that suchdevices have achieved satisfactory characteristics and the devices arenot positively fabricated. This is because it is difficult to controlmicrofabrication in a step such as implanting ions into a SiCsemiconductor substrate.

When a Si-based semiconductor substrate is used to fabricate asemiconductor device, p dopant and n dopant are selectively introducedthrough a single mask and are thermally diffused to implement precisechannel density. More specifically, JFET and similar semiconductordevices have characteristics depending on the channel's dimensions,which can significantly precisely be controlled, and increased yield ofJFET or similar semiconductor devices can be achieved.

In contrast, if a SiC semiconductor substrate is used to fabricate asemiconductor device, the substrate hardly allows dopant diffusion, ascompared with a Si-based semiconductor substrate, and it is difficult toprecisely control channel density and the like, as can be done forexample in a semiconductor device employing a Si-based semiconductorsubstrate. The device tends to increase for example in channelresistance, with significantly large variation. As such, suchcharacteristics of SiC semiconductor device as expected are at presentinsufficiently achieved.

Furthermore, if a SiC semiconductor substrate is used to fabricate asemiconductor device and ions are implanted to introduce dopant, thedopant activates at poor rate. To increase the rate, the ions may beimplanted at a high temperature of 300° C. or higher. This, however,prevents a resist film from being suitably used as a mask layer for ionimplantation. Furthermore, if silicon oxide film, polysilicon film orthe like is used as a mask layer, and exposed to high temperature, themask layer tends to crack, peel off and the like disadvantageously.

As described above when a SiC semiconductor substrate is used tofabricate a semiconductor device (also referred in the presentspecification as a “SiC device”) it is necessary to implant ions in anenvironment of high temperature to reduce damage to crystal.

Accordingly there is a demand for a material developed to be usable as amask layer used in implanting ions in an environment of hightemperature, and techniques are being developed in associated fields. Itshould be noted that a mask layer containing SiO₂ as material has aproperty that can ensure high energy implantation in an environment ofhigh temperature. Such a property is utilized and SiO₂ film is used as amask layer for ion implantation and subsequently thermal diffusion isperformed to form a sufficiently deep doped region, as disclosed inJapanese Patent Laying-Open No. 10-256173 and Power Device, Power ICHandbook, edited by the Institute of Electrical Engineers of Japan, HighPerformance and High Function Power Device and Power IC Investigationand Research Committee, Corona Publishing Co., Ltd., July 1996, pp.38-41.

For example, a silicon substrate is subjected to CVD to have the entiresurface with SiO₂ film thereon and subsequently photolithography isemployed to form a mask pattern. In the photolithography, the SiO₂ filmhas an entire surface provided with photoresist and exposed to lightonly at a portion to be provided with a hole. The photoresist is thusexposed, and the exposed portion is removed by development. Then on theremaining photoresist the underlying SiO₂ film is dry etched and thusopened and thereafter the photoresist is removed to obtain a maskpattern of SiO₂.

Subsequently, B or similar dopant ions are implanted in approximately1×10¹⁴ cm⁻². As the SiO₂ film serves as a mask, the dopant ions areimplanted only in the film's opening. In the ion implantation, ions ofdopant obtained by discharging AsH₃, PH₃, BF₂ or similar gas areaccelerated to several tens to several hundreds keV and thus implantedinto a substrate. Subsequently, thermal diffusion is performed to pushin the dopant and thereafter the SiO₂ film is dissolved withhydrofluoric acid and thus removed. Subsequently in a semiconductordevice fabrication process such thin-film deposition, photolithography,etching, and ion implantation are repeatedly performed.

If a SiC device is fabricated, however, a SiC semiconductor substratedoes not allow sufficient thermal diffusion of dopant. Accordingly, toachieve sufficiently deep dopant implantation, high energy needs to beapplied to implant ions, and if the mask layer of SiO₂ exceeds 1 μm inthickness, it is prone to cracking and not suitably used as a mask layerfor ion implantation.

By contrast, an oxide film containing SiO₂ that has a thickness of 1 μmor smaller can prevent small energy applied to implant ions. As such,the ions cannot be accelerated with high energy and are thus implantedto a depth of 0.3 μm at most. As such in general a depth of implantationof 0.6 to 1 μm required for a semiconductor device can hardly beachieved and SiO₂ cannot suitably be used as a mask for a SiCsemiconductor substrate.

Furthermore, if SiO₂ is used as material for a mask, a series ofcomplicated steps is required including employing CVD to deposit SiO₂film, photolithography with resist used, dry etching to provide the SiO₂film with an opening, implanting ions, and removing the SiO₂ film.Furthermore, CVD and dry etching require that the semiconductorsubstrate be introduced in a vacuumed reactor, resulting in poorfabrication efficiency.

Thus employing a mask layer of SiO₂ is accompanied by adisadvantageously limited depth of ion implantation. As such it is notpositively used in fabricating semiconductor devices employing a SiCsemiconductor substrate. Should it be used, a complicated process isstill required to overcome the issues as described above.

DISCLOSURE OF THE INVENTION

The present invention contemplates a method of fabricating semiconductordevices that allows ion implantation at high temperature with ionsaccelerated with high energy to help to introduce dopant in asemiconductor substrate, a SiC semiconductor substrate in particular, ata selected region to sufficient depth.

To achieve the above object the present method employs ion implantationto provide a semiconductor substrate at a surface thereof with a regionhaving dopant introduced therein, and includes the steps of: providingthe semiconductor substrate at a surface thereof with a mask layerincluding a polyimide resin film; and implanting dopant ions.

Furthermore the present method in another aspect employs ionimplantation to provide a semiconductor substrate at a surface thereofwith a region having dopant introduced therein, and includes the stepsof: providing the semiconductor substrate at a surface thereof with amask layer including a SiO₂ film and a thin metal film; and implantingdopant ions.

The semiconductor substrate can be a SiC semiconductor substrate. Thesemiconductor substrate at a region provided with the mask layer can beprevented from introduction of dopant ions so that the dopant ions canbe introduced into a region unmasked by the mask layer.

If the mask layer including the polyimide resin film is deposited, thesemiconductor substrate is heated preferably to 300° C. or higher, morepreferably 500° C. or higher, and dopant ions are thus implanted. Thepolyimide resin film is suitably formed of photosensitive polyimideresin film and preferably has a thickness of at least twice the depth ofdopant introduced into the semiconductor substrate at a region free ofthe polyimide resin film. Preferably the polyimide resin film and thesemiconductor substrate sandwich a thin film of metal or SiO₂.

In contrast, if the mask layer including the SiO₂ film and the thinmetal film is deposited, the semiconductor substrate is heatedpreferably to 300° C. to 500° C., more preferably 500° C. to 800° C.,and dopant ions are introduced. Suitably the mask layer is formed ofthree or more layers, and the SiO₂ resin film and the thin metal filmeach preferably have an average thickness of 500 nm to 1.5 μm. The masklayer preferably has the SiO₂ film or the thin metal film as a layercorresponding to a bottommost layer or that corresponding to a topmostlayer. Such SiO₂ film can preferably be formed by SOG.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1E show a process representing the present method offabricating semiconductor devices.

FIG. 2 represents a relationship between a polyimide resin film'sthickness and implanted dopant (Al)'s depth.

FIG. 3 is a cross section showing a polyimide resin film and a SiCsemiconductor substrate with a thin film posed therebetween in a mannerin accordance with the present invention.

FIGS. 4A-4E show a process representing the present method offabricating semiconductor devices.

BEST MODE FOR CARRYING OUT THE INVENTION

Method Employing Polyimide Resin Film as Mask to Fabricate SemiconductorDevice

The present method is characterized by including the step of providing asemiconductor substrate at a surface thereof with a mask layer includingpolyimide resin film, and subsequently implanting dopant ions. Thesemiconductor substrate is provided with the polyimide resin film, whichcan be used as a mask for the semiconductor substrate to allow ions tobe implanted at high temperature with high energy to introduce dopant sothat a SiC semiconductor substrate can also have dopant introduced tosufficient depth.

The present invention employs a semiconductor substrate which ispreferably, among other conventionally known semiconductor substrates, aSiC semiconductor substrate, since silicon carbide (SiC) provides a widebandgap, and a maximum insulating electric field larger than silicon(Si) by approximately one digit, and is comparable to Si in carriermobility and comparable to GaAs in electron saturation drift rate, andalso large in dielectric strength.

Furthermore, it is also because the present invention employs a masklayer, as will be described hereinafter, that exhibits an excellentproperty allowing high energy implantation in an environment of hightemperature and hence sufficiently deep dopant implantation into a SiCsemiconductor substrate and other semiconductor substrates allowingsmall thermal diffusion of dopant.

In the present specification a SiC semiconductor substrate refers to asemiconductor substrate containing SiC as material. Herein, the SiCsemiconductor substrate is not required to contain SiC alone as materialand may contain other components as material within a range that doesnot impair SiC's excellent property.

The present invention employs SiC having a crystal structure which isnot particularly limited, and may employ SiC for example havinghexagonal close packed structure or cubic close packed structure.Furthermore, SiC's hexagonal close packed structure further includes alarge number of structures different in layer repetition cycle and morethan 100 types of poly-type are known, and any type of structure may beused. For example, as representative poly-types, 3C, 4H, 6H and the likecan be used. In the present specification, “C” represents cubic crystaland “H” represents hexagonal crystal, and a preceding numeral representsa repetition cycle. Of these, for cubic system, 3C is the only one, andit is referred to as β-SiC while the others are generally referred to asα-SiC.

It should be noted, however, that the semiconductor substrate employedin the present invention is not limited to a SiC semiconductor substrateand may be any conventionally known semiconductor substrate, since thepresent invention employs a mask layer that allows dopant implantationat high temperature by ions of high energy and hence sufficiently deepdopant implantation if a semiconductor substrate other than a SiCsemiconductor substrate is used.

As a typical example of the present method a method employingphotosensitive polyimide resin film and implanting ions into a SiCsemiconductor substrate will be shown in FIGS. 1A-1E. Initially as shownin FIG. 1A a SiC semiconductor substrate 1 is provided thereon with aphotosensitive polyimide resin film 2. Subsequently, as shown in FIG.1B, a mask 3 having a prescribed pattern is introduced and therethroughthe intermediate product is exposed to light 4 and then developed andfired. Thus, as shown in FIG. 1C, on the SiC semiconductor substrate apolyimide resin film 2 a having the prescribed pattern can be readilyprovided.

Subsequently, as shown in FIG. 1D, through a mask layer includingpolyimide resin film, ions 5 are implanted into SiC semiconductorsubstrate 1 to provide a doped region 1 a. Finally, the polyimide resinfilm is removed to obtain SiC semiconductor substrate 1 having a dopedregion 1 a, as shown in FIG. 1E. Thus a prescribed mask can preventimplantation of dopant ions into a masked region and allows implantationof the dopant ions only at an unmasked region.

Dopant ions are preferably implanted with the SiC semiconductorsubstrate heated to 300° C. or higher, preferably 500° C. or higher, toprevent the SiC semiconductor substrate's crystal structure from beingamorphous. Furthermore, the substrate's temperature is preferably 1000°C. or lower, more preferably 800° C. or lower, to prevent SiC fromsublimation.

Polyimide is a condensation polymer composed of bifunctional carboxylicanhydride and primary diamine and having an imide structure (—CO—NR—CO)at a main chain of a polymer skeleton. Of polyimides, aromaticheterocyclic polyimide is preferable as it has an excellent physicalproperty and has significant stability against heat and oxidation.Furthermore, of aromatic heterocyclic polyimides, a polyimide derivedfrom aromatic diamine and aromatic dianhydride is more preferable as itis stable against heat.

Furthermore, the polyimide resin film is preferably formed ofphotosensitive polyimide resin as it can help to form a mask having aprescribed pattern or a SiC semiconductor substrate. Photosensitivepolyimide resin film can be deposited simply by applying it on the SiCsemiconductor substrate. It can thus eliminate the necessity ofperforming a complicated process including photolithography employingphotoresist, as employed when SiO₂ is used as material for a mask, andfacilitate implantation of ions into the SiC semiconductor substrate ata region selectively. Furthermore, CVD, dry etching and the like can bedispensed with and high fabrication efficiency can be achieved.

The SiC semiconductor substrate is provided thereon with a polyimideresin film preferably having a thickness twice or more the depth ofdopant introduced into the SiC semiconductor substrate at a region thatis not provided with the polyimide resin film. FIG. 2 represents arelationship between a polyimide resin film's thickness and implanteddopant (Al)'s depth, as provided when Al ions are implanted into a4H-SiC semiconductor substrate with acceleration energy of 340 keV in adosage of 1.0×10¹⁵ cm⁻².

As is apparent from the FIG. 2 result, a region without the polyimideresin film has the dopant introduced to a depth of 1.1 μm. In contrast,a region with a polyimide resin film having a thickness of 2.2 μm hasthe dopant introduced to a depth of 0.0 μm, or completely interrupts theAl ions. Considering the energy of ions to be implanted, and accordinglyforming a polyimide resin film to have a thickness of at least twice thedepth of a region to be doped, can completely interrupt ions to ensurethat ions are implanted only at a selected region.

Polyimide resin film is significantly adhesive and has high chemicalresistance, and to help to remove the polyimide resin film after ionimplantation, a thin metal or SiO₂ film 36 is preferably introducedbetween polyimide resin film 32 and SiC semiconductor substrate 31. Thethin film that is formed of Al or similar metal or SiO₂ has a thicknesspreferably of 0.02 μm or larger, more preferably 0.05 μm or larger asthe thin film can be wet etched to help to remove the polyimide resinfilm. On the other hand, such a thin film preferably has a thickness of0.5 μm or smaller, preferably 0.2 μm or smaller as it can be readilyetched away and side-etching can also be reduced. Accordingly, such athin film preferably has a thickness of approximately 0.1 μm.

Preferably the thin film formed for example of SiO₂ is provided on theSiC semiconductor substrate before the polyimide resin film is provided,and after the polyimide resin film is exposed, developed and fired thethin film that is located in an opening of the polyimide resin film iswet etched away. Ion implantation is not prevented by the thin film andcan proceed smoothly.

Method Employing SiO₂ film and Thin Metal Film as Mask to FabricateSemiconductor Device

Another present method includes the steps of providing a semiconductorsubstrate at a surface thereof with a mask layer including a SiO₂ filmand a thin metal film; and implanting dopant ions in a surface of thesemiconductor substrate. Such a mask also hardly cracks in a SiO₂containing mask layer for a thickness capable of preventing implantationof ions with high energy, and such a mask layer can be used to alsoallow implantation of ions at high temperature with high energy andhence sufficiently deep dopant implantation into a SiC semiconductorsubstrate and other semiconductor substrates allowing small thermaldiffusion of dopant.

FIGS. 4A-4E show a process illustrating the present method. Initially, asemiconductor substrate is provided on a surface thereof with a masklayer formed of a composite film including a SiO₂ film and a thin metalfilm. FIG. 4A shows the present method at the step of depositing a masklayer 103. In accordance with the present invention a semiconductordevice 1000, as shown in FIG. 4A, has a semiconductor substrate 101, andon a surface thereof mask layer 103 is deposited. Semiconductorsubstrate 101 including SiC and other semiconductors is as has beendescribed previously. Furthermore in the FIG. 4A example mask layer 103is a composite film having structure formed of three layers including aSiO₂ film 107 a, a thin metal film 105 and a SiO₂ film 107 b.

Generally in fabricating a semiconductor device it is important thatdopant be introduced only at a predetermined region selectively. One ofmeans allowing selective dopant introduction is ion implantation througha mask layer. For semiconductor devices formed of SiC semiconductor orsimilar semiconductor allowing small thermal diffusion of dopant, inparticular, ion implantation through a mask layer is almost the onlypractical means to selectively introduced dopant. By forming aprescribed mask a masked region can be prevented from dopant ionimplantation and an unmasked region alone is allowed to have dopant ionimplantation.

In the present method a mask layer deposited on a semiconductorsubstrate to select a region to have ions implanted therein can beprovided in the form of a composite film of SiO₂ film and thin metalfilm as shown in FIG. 4A to allow region-selective ion implantation intoa SiC semiconductor substrate and other semiconductor substratesallowing small thermal diffusion of dopant, while reducing damage tocrystal structure.

The present invention's mask layer is a mask layer used in implantingdopant ions into a semiconductor substrate and includes a SiO₂ film anda thin metal film. The SiO₂ film may be any oxide film that containsSiO₂ as material, since such oxide film has an excellent property thatcan endure high energy implantation in an environment of hightemperature. Furthermore, the SiO₂ film is not required to contain SiO₂as the only material and may contain other components within a rangethat does not impair SiO₂'s excellent property.

The mask layer includes the SiO₂ film, which is not particularly limitedand may be provided by conventionally known methods. For example, SOGcan be employed to deposit the SiO₂ film. More specifically, silanol[(OR)_(m)R_(n)Si(OH)_(4-m-n)] dissolved in alcohol or similar solvent isspin-applied on a wafer and then thermally set to obtain an insulationfilm close in composition to pure SiO₂ (also referred to in the presentspecification as SOG film). In the present specification the SiO₂ filmincludes SOG film. The SOG film includes inorganic and organic SOG filmsdepending on the type of silanol compound. The SOG method, utilizingliquid to deposit film, is advantageous in that it can fill a narrowspace between interconnects.

The present invention's mask layer includes the SiO₂ film having anaverage thickness preferably of 500 nm or larger, more preferably 800 nmor larger. Furthermore, an average thickness of 1.5 μm or smaller ispreferable, and an average thickness of 1.2 μm or smaller is morepreferable. If the SiO₂ film has an average thickness of less than 500nm, the film can prevent only limited energy of ion implantation,providing a tendency that ions are implanted shallower. If the SiO₂ filmhas an average thickness exceeding 1.5 μm, the film is prone to crackingin an environment of high temperature.

The mask layer may include any thin metal film that contains metal asmaterial, although metal vapor-deposited film is particularlypreferable. The metal vapor-deposited film is readily obtained by vapordepositing a metal by a conventionally known method on a SiO₂ containingoxide film, a SiC semiconductor substrate or the like. The thin metalfilm deposited through metal vapor deposition is preferably provided forexample through EB vapor deposition. By including the metal vapordeposited film and other thin metal film in the mask layer, the SiO₂film does not have an average thickness exceeding 1.5 μm and the masklayer in its entirety can have an average thickness of 1.5 μm or larger.As such if the SiO₂ film is exposed to high temperature it hardly cracksand high energy ion implantation can be prevented.

The mask layer's thin metal film is not particularly limited and may bea thin film containing any conventionally known metal as material. Forexample, a thin film for example containing aluminum, nickel, gold orsimilar metal as material can be used. Of these metals, aluminum isparticularly preferably contained as a material in the thin film as sucha thin film can be provided readily and inexpensively. It is not arequirement that the thin metal film include metal as the only material.It may contain other components as material within a range that does notimpair the thin metal film's excellent property.

The mask layer's thin metal film preferably has an average thickness of500 nm or larger, more preferably 800 nm or larger. Furthermore, anaverage thickness of 1.5 μm or smaller is preferable, and an averagethickness of 1.2 μm or smaller is more preferable. If the thin metalfilm has an average thickness less than 500 nm, then in an environmentof high temperature the SiO₂ film is prone to cracking and high energyion implantation tends to be difficult to achieve. If the thin metalfilm has an average thickness larger than 1.5 μm then side-etching tendsto increase in patterning the mask.

The present invention's mask layer is a mask layer employed inimplanting dopant ions into a semiconductor substrate, and it may have astructure formed of two or more layers that are implemented by SiO₂ filmand thin metal film. The structure formed of three or more layers canprevent the SiO₂ film exposed to high temperature from being prone tocracking, and the mask layer having an increased total thickness canprevent high energy ion implantation.

The present invention's mask layer preferably includes SiO₂ film as afilm corresponding to a bottommost layer. Such a structure can preventmetal ions originated from the thin metal film from contaminating SiCand other semiconductor substrates. Furthermore the mask layerpreferably includes a thin metal film as a film corresponding to abottommost layer. Such structure can help to remove the mask layer fromthe semiconductor substrate after ion implantation.

The present invention's mask layer preferably includes SiO₂ film as afilm corresponding to a topmost layer. Such a structure can prevent themetal vapor deposited film and other thin metal films from being etchedaway for example by reactive ion etching (RIE) and can thus help to forma pattern. Furthermore, the mask layer preferably includes a thin metalfilm as a film corresponding to a topmost layer. Such structure canminimize effect of crack introduced in the SiO₂ film.

Among these structures, the present invention's mask layer particularlypreferably has a structure including, as seen from the bottommost layer,a SiO₂ film, a thin metal film and a SiO₂ film sequentially. The masklayer having such a 3-layer structure does not have a SiO₂ film havingan average thickness exceeding 1.5 μm, and the mask layer can have atotal average thickness of 1.5 μm or larger. As such, the SiO₂ filmhardly cracks even in an environment of high temperature and the masklayer in its entirety can prevent high energy ion implantation.

Then for example as shown in FIG. 4B semiconductor device 1000 providedwith mask layer 103, as described with reference to FIG. 4A, has resistmaterial applied on mask layer 107 a and a glass mask 111 is then usedfor patterned exposure to set the resist material to provide a resistfilm 109. The resist material is not particularly limited andconventionally known resist material can be selected to meet thecondition of interest. Furthermore, the glass mask is also notparticularly limited and conventionally known glass mask can be used toprovide patterned exposure.

Subsequently for example as shown in FIG. 4C semiconductor device 1000provided with resist film 109, as described with reference to FIG. 4B,is RIEed, wet etched or the like etching with resist film 109 used toform a patterned mask layer 103 a. RIE or the like can be performedunder conventionally known conditions. For example, parallel plate RIEapparatus, acidic solution, and the like can be employed.

Subsequently as shown in FIG. 4D semiconductor substrate 1000 issubjected to ion implantation to introduce dopant into SiC semiconductorsubstrate 101. The present invention can employ any type of dopant, andit can be selected, as appropriate, depending on the structure andobject of the semiconductor device fabricated. For example, aluminum,boron, nitrogen, phosphorus and the like can be selected. Furthermore,ions can be implanted under conventionally known conditions, althoughpreferably by employing a high current ion implantation apparatus or thelike.

In the present invention ions are preferably implanted in a dosage of1×10¹⁵ cm⁻² or less. For a dosage exceeding 1×10¹⁵ cm⁻², implanted ionstend to collide with and thus push previously implanted ions deeper.Furthermore, for a dosage exceeding 1×10¹⁷ cm⁻², SiC's crystal readilydestroys and thus becomes amorphous.

In accordance with the present invention when ions are implanted into asubstrate the substrate desirably has a temperature of 300° C. orhigher, particularly desirably 500° C. or higher to reduce damage by ionimplantation to a SiC semiconductor substrate's crystal structure.Furthermore, desirably the substrate has a temperature of 1000° C. orlower, 800° C. or lower in particular, to prevent sublimation of SiC. Inaccordance with the present invention ions are implanted at any angleapplied in conventionally known ion implantation methods. Preferably,however, ions are implanted for example at an angle perpendicular to thesubstrate.

Subsequently, as shown in FIG. 4E, semiconductor device 1000 havingdopant ion-implanted, as described with reference to FIG. 4D, has masklayer 103 a removed therefrom. The mask layer may be removed by anyconventionally known method therefor. Preferably, however, it is forexample dissolved with acidic solution and thus removed.

In accordance with the present invention a semiconductor substrate isdoped with ions implanted by a method including the steps of: providingthe semiconductor substrate at a surface thereof with a mask layerincluding a SiO₂ film and a thin metal film; and implanting ions ofdopant into the surface of the semiconductor substrate. The details ofthis method is similar to those of the present method of fabricatingsemiconductor devices.

FIRST EXAMPLE

Initially, as shown in FIG. 1A, a 4H-SiC semiconductor substrate 1 of 5inches in diameter and 600 μm in thickness is spin-coated with negativephotosensitive polyimide resin (HD4010 produced by Hitachi-DupontMicrosystems) and dried in the atmosphere to form a photosensitivepolyimide resin film 2 having a thickness of 3.0 μm. Then, as shown inFIG. 1B, through mask 3 having a prescribed pattern, photosensitivepolyimide resin film 2 is exposed to light 4 and then developed with adedicated developer formed of organic solvent, and then fired to form apatterned polyimide resin film 2 a on the SiC semiconductor substrate ata region to be undoped, as shown in FIG. 1C. The use of thephotosensitive polyimide resin allows the mask to be formed more readilythan photolithography.

Subsequently, SiC semiconductor substrate 1 and the polyimide resin filmare heated to 500° C., and, as shown in FIG. 1D, through patternedpolyimide resin film 2 a Al ions 5 are implanted into SiC semiconductorsubstrate 1 to form a doped region 1 a. The Al ions are implanted withacceleration energy of 340 keV and in a dosage of 1.0×10¹⁵ cm⁻².Finally, the polyimide resin film is removed with hydrofluoric acid toobtain SiC semiconductor substrate 1 having doped region 1 a, as shownin FIG. 1E, having a depth of 1.1 μm. A doped region having a depth thatcannot be achieved with conventional SiO₂ mask can thus be obtained.

SECOND EXAMPLE

As shown in FIG. 3, a SiC semiconductor substrate having a doped regionis produced similarly as has been described in the first example exceptthat a thin metal film 36 formed of Al and having a thickness of 0.1 μmis posed between a polyimide resin film 32 and SiC semiconductorsubstrate 31. After ion implantation the polyimide resin film providedon the SiC semiconductor substrate with the thin metal film of Al posedtherebetween can be readily wet etched away from the SiC semiconductorsubstrate with phosphoric acid as the thin metal film of Al serves as aboundary. Increased production efficiency can thus be achieved.

The thin film of Al is provided on the SiC semiconductor substratebefore the polyimide resin film is provided, and after the polyimideresin film is exposed, developed and fired the thin film that is locatedin an opening of the polyimide resin film is wet etched away. Ionimplantation is thus not prevented.

THIRD EXAMPLE

A SiC semiconductor substrate having a doped region is producedsimilarly as has been described in the first example except that a thinfilm formed of SiO₂ and having a thickness of 0.1 μm is posed between apolyimide resin film and the SiC semiconductor substrate. After ionimplantation, the polyimide resin film is wet-etched away withhydrofluoric acid. The polyimide resin film is readily removed,similarly as has been described in the second example, as the thin filmof SiO₂ film serves as a boundary. Operation can be conductedefficiently.

The thin film of SiO₂ is provided on the SiC semiconductor substratebefore the polyimide resin film is provided, and after the polyimideresin film is exposed, developed and fired the thin film formed of SiO₂that is located in an opening of the polyimide resin film is wet etchedaway. Ion implantation is thus not prevented.

FOURTH EXAMPLE

Initially a 1 cm square 4H-SiC substrate is prepared (having a surfacewith an orientation inclined relative to a 0001 plane by approximately8°). Then, as shown in FIG. 4A, SiC semiconductor substrate 101 has, asseen upwards, a SiO₂ film 107 b (average thickness: 1 μm), a thin metalfilm of Al 105 (average thickness: 1 μm), and a SiO₂ film 107 a (averagethickness: 1 μm) deposited thereon sequentially to together form masklayer 103. Note that SiO₂ films 107 a and 107 b are deposited by SOG andthin metal film of Al 105 is provided through metal vapor deposition.

Then, as shown in FIG. 4B, SiO₂ film 107 a has resist material appliedthereon and glass mask 111 is then used for patterned exposure to setthe resist material to provide resist film 109. Then SiC semiconductorsubstrate 101 with resist film 109 is RIEed with resist film 109interposed to form patterned mask layer 103 a (FIG. 4C). Then, as shownin FIG. 4D, through mask layer 103 a into SiC semiconductor substrate101 ion implantation is performed to introduce Al ions as dopant intoSiC semiconductor substrate 101.

In the ion implantation, the SiC semiconductor substrate and the masklayer are previously heated to 500° C. and the Al ions are implantedwith acceleration energy 340 keV and in a dosage of 1×10¹⁵ cm⁻².Finally, SiC semiconductor substrate 101 having dopant ions implantedtherein as described with reference to FIG. 4D is exposed tohydrofluoric acid to dissolve and thus remove patterned mask layer 103therefrom (FIG. 4E).

SiC semiconductor substrate 101 thus obtained is then annealed in Ar at1700° C. for 30 minutes and then evaluated by SIMS. It has been foundthat a region covered with patterned mask layer 103 a does not have Aldetected therein and Al implantation with acceleration energy of 340 keVcan thus be prevented. A doped region 115 has a depth of 1.1 μm, whichis a depth that cannot be achieved with conventional SiO₂ mask.Furthermore, as evaluated by Raman scattering measurement, it has beenfound that the crystal structure of SiC semiconductor substrate 101 hasnot been damaged.

FIRST COMPARATIVE EXAMPLE

A SiC semiconductor substrate is subjected to ion implantation similarlyas has been described in the fourth example except that the SiCsemiconductor substrate has deposited thereon a mask layer formed onlyof SiO₂ film (average thickness: 1 μm). The SiC semiconductor substratehaving dopant ions implanted therein is evaluated similarly as has beendescribed in the fourth example, and it has been found that the masklayer formed only of SiO₂ film (average thickness: 1 μm) is insufficientin thickness, and a region covered with the mask layer formed only ofSiO₂ film (average thickness: 1 μm) also has Al detected therein and Alion implantation with acceleration energy of 340 keV is insufficientlyprevented. SiC semiconductor substrate 101 has its crystal structureundamaged.

SECOND COMPARATIVE EXAMPLE

A SiC semiconductor substrate is subjected to Al ion implantationsimilarly as has been described in the fourth example except that theSiC semiconductor substrate has deposited thereon a mask layer formedonly of SiO₂ film (average thickness: 3 μm). The SiC semiconductorsubstrate having dopant ions implanted therein is evaluated similarly ashas been described in the fourth example, and it has been found that themask layer formed of SiO₂ film (average thickness: 3 μm) has a crack andtherein Al is detected, and the mask insufficiently prevents Al ionimplantation with acceleration energy of 340 keV. The SiC semiconductorsubstrate 101 has its crystal structure undamaged.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

INDUSTRIAL APPLICABILITY

The present invention can reduce damage to a crystal structure whilefacilitating region-selective, high energy ion implantation into asurface of SiC and other semiconductor substrates to allow sufficientlydeep dopant implantation into the substrate. Furthermore there can alsobe provided a mask layer that does not crack if it is exposed to hightemperature.

1. A method of fabricating a semiconductor device by employing ionimplantation to provide a SiC semiconductor substrate at a surfacethereof with a region having dopant introduced therein, comprising thesteps of: providing said semiconductor substrate at least at a firstregion of a surface thereof with a mask layer including a polyimideresin film; and implanting dopant ions to an implantation depth intosaid semiconductor substrate at a second region of said surface of saidsemiconductor substrate free of said polyimide resin film; wherein saidpolyimide resin film has a thickness of at least twice said implantationdepth.
 2. The method of claim 1, wherein said semiconductor substrate isheated to at least 300° C., and said dopant ions are implanted.
 3. Themethod of claim 1, wherein said semiconductor substrate is heated to atleast 500° C. and said dopant ions are implanted.
 4. The method of claim1, wherein said polyimide resin film is formed of photosensitivepolyimide resin.
 5. The method of claim 1, wherein a thin metal film isfurther interposed between said polyimide resin film and saidsemiconductor substrate.
 6. The method of claim 1, wherein a thin filmformed of SiO₂ is further interposed between said polyimide resin filmand said semiconductor substrate.
 7. The method of claim 1, wherein saidmask layer is deposited on said semiconductor substrate at said firstregion to be undoped with said dopant ions.
 8. The method of claim 1,wherein said a dopant ions are implanted into said second region whichis not masked by said mask layer.
 9. A method of preparing a dopedsemiconductor substrate, comprising the steps: a) providing asemiconductor substrate comprising SiC; b) providing a mask layerincluding a polyimide resin film that consists of a photosensitivepolyimide resin on a first region of a surface of said substrate, byapplying said polyimide resin film on said first region and a secondregion of said surface, then exposing said polyimide resin film to lightat said first region, and then removing said polyimide resin film atsaid second region, wherein said step b) does not involvephotolithography employing a photoresist; c) heating said substrate toat least 300° C.; and d) while said substrate is at least 300° C.,implanting, by ion implantation, dopant ions into said substrate throughsaid second region of said surface to form in said substrate a dopedregion that is doped with said dopant ions; wherein said method does notinvolve chemical vapor deposition and does not involve dry etching. 10.The method according to claim 9, further comprising, after said step d),a step of removing said polyimide resin film by wet etching usinghydrofluoric acid.
 11. A method of preparing a doped semiconductorsubstrate, comprising the steps: a) providing a semiconductor substratecomprising SiC; b) providing a mask layer including a polyimide resinfilm on a first region of a surface of said substrate; c) heating saidsubstrate to at least 300° C.; and d) while said substrate is at least300° C., implanting, by ion implantation, dopant ions into saidsubstrate through a second region of said surface to form in saidsubstrate a doped region that is doped with said dopant ions; whereinsaid polyimide resin film has a thickness of at least twice a depth ofsaid doped region.
 12. The method according to claim 11, wherein saidstep b) further comprises forming said mask layer to include a metalfilm interposed between said polyimide resin film and said substrate.13. The method according to claim 11, wherein said step b) furthercomprises forming said mask layer to include a SiO₂ film interposedbetween said polyimide resin film and said substrate.
 14. The methodaccording to claim 9, wherein said step b) further comprises formingsaid mask layer to include a metal film interposed between saidpolyimide resin film and said substrate.
 15. The method according toclaim 9, wherein said step b) further comprises forming said mask layerto include a SiO₂ film interposed between said polyimide resin film andsaid substrate.